1. Field of the invention
The present invention relates to the process of producing various organic carbonates by performing chemical reactions, which are limited by equilibrium, and separating various chemical compounds involved. Achieving higher conversion than equilibrium condition is highly desirable for better economic reward. This invention discloses a method of shifting the equilibrium position of a chemical reaction to achieve higher conversion.
2. Related information
Diaryl carbonates, for example diphenyl carbonate (DPC), are an important raw material for the production of polycarbonates. The current state of the art produces DPC from dimethyl carbonate (DMC) and phenol in two steps by employing multiple reactors and a homogeneous catalyst, such as titanium alkoxide. The following three reactions are involved in producing DPC.
The equilibrium for each of the above reactions lies on far left side, whereas it is an object of the present invention that the equilibriums are moved to the right.
J. L. R. Williams et al. (J. Org. Chem., 24 (1) pp. 64-68, 1959) discovered that disproportionation of unsymmetrical carbonates such as methyl phenyl carbonate (MPC) and symmetrical carbonates such as dibenzyl carbonate can be performed in the presence of a suitable homogeneous catalyst, particularly metal alkoxide. However, the reaction accompanies a number of undesired side reactions such as decomposition of carbonate compounds to carbon dioxide, polymerization, formation of olefins, and ethers. The authors conclude that the course of the reactions depends on the structure of the carbonates and on the catalyst and the more alkaline catalysts promote more side reactions for the disproportionation of alkyl phenyl carbonates than mildly acidic catalyst such as titanium butoxide. Alkyl phenyl ether is the major undesired by-product.
U.S. Pat. No. 4,045,464 (1977) discloses a process for preparing diaryl carbonates from phenyl alkyl carbonates via the Reaction (3) in the presence of homogeneous Lewis acid catalysts of the formula AlX3, TiX3, UX4, TiX4, VOX3, VX5, and SnX4, where X is a halogen, acetoxy, alkoxy or aryloxy group. DPC was produced by performing the disproportionation of ethyl phenyl (EPC) or MPC at 180° C. with 95% selectivity in the presence of homogeneous titanium catalysts.
U.S. Pat. No. 4,554,110 (1985) discloses an improved process for the preparation of aromatic carbonates from a dialkyl carbonate and phenol in the presence of catalyst comprising polymeric tin compounds. Diaryl carbonates are prepared by performing disproportionation of alky aryl carbonate.
U.S. Pat. No. 5,210,268 (1993) discloses a process for producing diaryl carbonates by performing various transesterification reactions in two reaction zones. An aromatic carbonate mixture is prepared by a transesterification reaction between a dialkyl carbonate, an alkyl aryl carbonate and a mixture thereof, and an aromatic hydroxy compound in the first step. A diaryl carbonate is produced by primarily performing disproportionation of alkyl aryl carbonate in the second step. The patent discloses the method of overcoming unfavorable equilibrium. The transesterification is performed in reactive distillation mode by feeding reactants to continuous multistage distillation columns in the presence of a homogeneous catalyst or a solid catalyst, while continuously withdrawing the produced aromatic carbonate mixture as a high boiling point product from a lower portion of the distillation column and continuously withdrawing light coproducts such as aliphatic alcohol or dialkyl carbonate as vapor stream from upper portion of the distillation columns. The disproportionation of alkyl aryl carbonate to diaryl carbonate and dialkyl carbonate is also performed in similar fashion. Diaryl carbonate is continuously produced by performing transesterification and disproportionation in sequence, utilizing multiple multi-stage distillation columns.
U.S. Pat. Nos. 5,872,275 (1999) and 6,262,210 (2001) disclose for the process producing diary carbonate from dialkyl carbonate and an aromatic hydroxy compound in the presence of liquid homogeneous catalyst and methods removing heavy high boiling point byproducts and regenerating catalyst for recycle.
U.S. Pat. No. 6,093,842 (2000) discloses the process producing diaryl carbonate from a dialkyl carbonate, an aromatic hydroxy compound and a mixed solution containing alkyl aryl carbonate by introducing three reactant streams into an extractive/reactive distillation column in the presence of a catalyst. Examples of the catalysts are lead compounds, copper compounds, alkali metal compounds, nickel compounds, zirconium compounds, titanium compounds, vanadium compounds, etc. The by-products are CO2, anisole, benzoates and heavy materials.
U.S. Pat. No. 5,426,207 (1995) discloses the process producing a diaryl carbonate such as DPC by conducting transesterification of DMC with an aromatic hydroxy compound and disproportionation of alkyl aryl carbonate in presence of a homogeneous catalyst in three successive reaction zones. Conditions are selected to maximize formation of alkyl aryl carbonate in the first and second reaction zones, while disproportionation is favored in the third reaction zone.
U.S. Pat. No. 6,767,517 (2004) discloses a process for the continuous production of diaryl carbonates. The process uses three reactive distillation column reactors and two rectification columns for the separation of intermediate reaction product and final product.
To alleviate shortcomings associated with using homogeneous catalysts for the production of diaryl carbonates, U.S. Pat. Nos. 5,354,923 (1994) and 5,565,605 (1996), and WO 03/066569 (2003) disclose heterogeneous catalysts.
WO 03/066569 discloses a process continuously producing an aromatic carbonate such as DPC in the presence of a heterogeneous catalyst prepared by supporting titanium oxide on silica (3 mm diameter) in two steps.
JP 79,125,617 (1979) and JP Application NO HEI 07[1995]-6682 discloses heterogeneous catalysts for the preparation of diphenyl carbonate by transesterification of DMC with phenol to MPC and disproportionation of MPC to DPC in the presence of MoO3 or V2O5 supported on an inorganic support such as silica, zirconia or titania. The transesterification and disproportionation are carried out in a reactor-distillation tower consisting of a reactor and distillation tower with removal of the co-products by distillation.
The publication by Z.-H. Fu et al. (J Mol. Catal. A: Chemical 118, (1997) pp. 293-299) reports the synthesis of diphenyl carbonate from DMC and phenol in the presence of various heterogeneous metal oxide catalysts. The best selectivity is reported for the MoO3 (20 wt. % optimum loading) catalyst supported on silica.
Due to many shortcomings of current DPC processes, an improved process is highly desired for saving materials, cheaper construction cost, consuming less energy, and plant operation cost. Although the prior art does not address catalyst life, all heterogeneous catalysts eventually deactivate and become useless. Heterogeneous catalysts are only practical in commercial use, if their cycle and service times are long enough or the catalysts can be rejuvenated insitu without serious financial cost. Thus, the issues of the catalyst deactivation and method of regeneration remain as substantial bars to the commercial application of the prior art.